The proposed research is concerned with elucidating the mechanism whereby the immune response shifts from IgM to IgG antibody production. The available evidence indicates that this transition is influenced by thymus derived (T) lymphocytes. The objectives of the proposed research are (a) establish that T lymphocytes do indeed influence switchover; (b) ascertain the general mechanism through which it occurs (e.g. cell contact vs. soluble mediators, role of macrophages, role of cell surface receptors, etc.); and (c) attempt to develop methods whereby the switchover effect can be both reversibly enhanced and blocked. The general approach to the problem will deal with the effect of non- specifically generated T lymphocyte function on the antibody response to antigens which normally induce IgM antibodies only. The experiments will be carried out both in vivo and in vitro, the latter including utilization of methods which prevent physical contact between given cell populations while allowing cross-contact of their various extracellular products.